Enum literals assignments are inferred differently than what is emitted in .d.ts.

[rkirov]

TypeScript Version: 3.1.0-dev.20180726

Search Terms:

enum literals declarations

Code

enum E {
  A = 'a',
  B = 'b'
}

class C {
  readonly type = E.A;
}

let x: E.A = new C().type; // no error, type is inferred to E.A, not E.

Expected behavior:
I expect similarly the .d.ts emitted to have a narrowed declaration for 'type'

declare class C {
    readonly type: E.A;
}

Actual behavior:
Emitted .d.ts contains:

declare class C {
    readonly type: E;
}

Note this is critical for creating tagged unions for type narrowing, and this behavior makes type narrowing only work when one is not crossing a '.d.ts' boundary. Bazel compliations usually look like 2-3 .ts files and hundreds to thousands .d.ts files from downstream compilations.

Related Issues:

[mhegazy]

Duplicate of #15881

[weswigham]

Not quite a duplicate - enum references aren't currently allowed as constant initializers in declaration files.

[rkirov]

Thanks for picking the up. We are seeing a lot of code using this pattern because it is what ngrx (redux for angular) is recommending - https://github.com/ngrx/platform/blob/master/docs/store/actions.md#typed-actions